1. Field of the Invention
Embodiments of the present invention relate to a surface emitting laser and an image forming apparatus including the surface emitting laser.
2. Description of the Related Art
Vertical-cavity surface emitting lasers (VCSEL) are used as light sources of exposure optical systems included in electrophotographic image forming apparatuses.
In order to improve the performance of the surface emitting laser, the surface emitting laser having a stepped structure provided on an emitting surface and in an emission area has been developed.
For instance, in order to accomplish laser oscillation in single transverse mode, JP No. 2001-284722 A discloses a surface emitting laser having a stepped structure provided in its surface so that a reflectivity of a center portion is higher than that of a peripheral portion in an upper mirror.
Moreover, Japanese Patent Laid-Open No. 2001-284722 discloses a method forming the stepped structure by etching process or liftoff process.
When the stepped structure is provided on an emitting surface and in an emission area, a distribution of electric-field amplitude in a near field is modulated. Thus, a distribution of electric-field amplitude in a far field is also modulated. As a result, a distribution of an intensity in the far field (FFP: Far Field Pattern) is also modulated.
When a protective film is not formed on the stepped structure, a boundary between the stepped structure and air may be referred to as a lateral face. The lateral face may be called edge portion. A lateral face of the stepped structure is not always perpendicular. The lateral face may have some angular degree and the stepped structure may have a tapered shape.
The angular degree depends on how to manufacture it, but in general, controlling the angular degree is more difficult than that of a film thickness. Thus, it is difficult to make the FFP a predetermined value because a shape of the stepped structure may be varied.